My laboratory has demonstrated that the transition of quiescent BALB/c-3T3 cells from growth arrest into S phase can be separated into two stages under the control of separate serum growth hormones. The first stage--competence--is regulated by the PDGF component of serum. The second stage--progression--is regulated by somatomedins and other cofactors contained in platelet-poor plasma; progression requires both RNA and protein synthesis. In contrast to serum growth factors which provide either competence or progression activities (but never both), SV40 infection provides both competence and progression activities. The proposed research has three objectives. First, we wish to define the separate roles of somatomedins and plasma cofactors in controlling the progression of BALB/c-3T3 cells through G1 and S phase. Second, we wish to establish functional analogies between the transformation proteins of animal tumor viruses and the serum growth factors which regulate competence and progression. As a third objective, we wish to study regulation of RNA and protein metabolism by growth factors and virus transformation proteins using in vitro systems. This year we have isolated molecular clones of gene sequences which are induced 10-\to 20-fold in 3T3 cells within 1 hour following exposure to PDGF. Pure PDGF induces these gene sequences at 10 ng/ml or less whereas other mitogens (EGF, insulin) have no effect. In the fully induced state, these sequences constitute 0.05% to 0.5% of the poly A+ mRNA within BALB/c-3T3 cells. Preliminary screening of 8,000 clones from a cDNA library prepared to PDGF-treated 3T3 cells indicates that at least six and probably no more than 13 different mRNAs are induced by PDGF. Induction of these gene sequences is unaffected by cycloheximide; this result indicates that the clone inserts correspond to "early genes" which are not induced during the course of cell growth but rather are regulated directly by PDGF. One gene sequence is, in fact, "super-induced" more than 200-fold by the combination of PDGF and cycloheximide; cycloheximide alone has no specific effect. The super-induction phenomenon suggests that expression of this gene sequence is subject to feedback inhibition. Preliminary screening data indicates that the PDGF-induced gene sequences are not related to any of the known viral oncogenes.